The valve characteristic varying apparatus includes a variable valve timing controller which varies a valve timing of an intake valve and/or an exhaust valve by varying a rotational phase of a camshaft relative to a crankshaft of the engine. The variable valve timing controller includes a first rotational element rotating along with the crankshaft and a second rotational element rotating along with the camshaft. One of these rotational elements is accommodated in the other rotational element to define an oil chamber therebetween. Operation fluid (operation oil) is introduced into the oil chamber. The amount of the operation fluid in the oil chamber is controlled by an oil control valve (OCV) to adjust the rotational phase difference between the first rotational element and a second rotational element.
A response characteristic of the variable valve timing controller disperses due to an individual difference and aging of the OCV and a dispersion of fluid characteristic of the operation oil. Especially, when the engine is in a cold state, viscosity of the operation oil is high and frictional resistance in the variable valve timing controller becomes high. Hence, the response characteristic of the variable valve timing controller may deteriorate, and an adjustable range of the variation speed of the valve timing may become narrower.
JP-2003-254017A (U.S. Pat. No. 6,755,165B2) shows an inching control in which a drive signal and stop signal are alternately compulsorily applied to the OCV while the engine in the cold state, whereby a responsiveness of the variable valve timing controller is improved.
In performing the inching control, it is necessary to adapt the repeating periods of the compulsory drive signal and stop signal and length of each signal, which make the control complicated.